This invention relates to a pulsed Hall thruster system and more particularly to a such system in which either the propellant flow or the electrical power or both is pulsed.
Pulsed Hall thruster systems can be used for: propulsion of small spacecraft which lack sufficient power for steady state operation; attitude control in large spacecraft; and in spacecraft where steady state propulsion and/or delivery of small impulse bits are needed. One type of competing pulsed system, a cold gas thruster, uses pressurized gas expanded through a nozzle to create thrust. Such systems suffer from low specific impulse or thrust per unit mass flow. One such cold gas thruster is the Model 50-673 cold gas thruster triad available from Moog Space Products Division, East Aurora, N.Y. Another pulsed system, a pulsed plasma thrust (PPT) system employs an electrical arc to ablate a Teflon surface to create and accelerate a gasified Teflon. This suffers from low efficiency, potential for spacecraft contamination and produces impulse bits with low uniformity. Mueller, Juergen, xe2x80x9cThruster Options for Microspacecraft: A Review and Exaluation of State-of-the Art and Emerging Technologiesxe2x80x9d, Micropropulsion for Small Spacecraft, Edited by Michael M. Micci and Andrew D. Ketsdever, AIAA Progress in Astronautics and Aeronautics Vol. 187. See also: Spanjers, Gregory G., McFall, Keith A., Gulczinski III, Frank S., and Spores, Ronald A., xe2x80x9cInvestigation of Propellant Inefficiencies in a Pulsed Plasma Thrusterxe2x80x9d, Paper AIAA-96-2723, Joint Propulsion Conference, Orlando, Fla., Jul. 1-3, 1996; Hruby, V., Pote, B., Gamero-Castano, M. Kolencik, G., Byrne, L., Tedrake, R., and Delichatsios, M., xe2x80x9cHall Thrusters Operating in Pulsed Modexe2x80x9d, IEPC-01-66, International Electric Propulsion Conference, Pasadena Calif., Oct. 15-19, 2001; and U.S. Pat. No. 6,150,764 to Hruby et al. entitled xe2x80x9cTandem Hall Field Plasma Acceleratorxe2x80x9d.
Typically, Hall thrusters are started by establishing the magnetic field and then applying the starting voltage which typically exceeds the steady state discharge voltage. This results in hard starts, high initial current spikes and often non-repeatable discharge initiation: therefore they are not perceived as likely pulsed devices which could produce precisely controlled, repetitive impulses.
It is therefore an object of this invention to provide a Hall thruster system capable of both pulsed and steady state operation.
It is a further object of this invention to provide such a pulsed Hall thruster system which precisely controls impulses.
It is a further object of this invention to provide such a pulsed Hall thruster system which produces variable impulses.
It is a further object of this invention to provide such a pulsed Hall thruster system which can produce very small impulses for accurate spacecraft positioning and attitude control.
It is a further object of this invention to provide such a pulsed Hall thruster system which provides discrete or repetitive impulses.
It is a further object of this invention to provide such a pulsed Hall thruster system which can operate in steady state or pulse mode.
It is a further object of this invention to provide such a pulsed Hall thruster system which has high efficiency and high specific impulse.
It is a further object of this invention to provide such a pulsed Hall thruster system which can be powered by a capacitor, power processing unit, or even directly from a solar photovoltaic array or other power sources.
The invention results from the realization that a pulsed Hall thruster system which can vary yet precisely control discrete and repetitive impulses with high efficiency and specific impulse can be achieved by pulsing either the power to the Hall thruster or the propellant flow to the thruster discharge chamber with a duration of xcfx84 where xcfx84 less than 0.1d3xcfx81/{dot over (m)} as defined hereinafter or by pulsing both power and flow for approximately the same time and having them appear coincidentally at the discharge chamber.
This invention features a pulsed Hall thruster system including a Hall thruster having an operating electron source, a magnetic circuit and a discharge chamber. There is a power processing circuit for firing the Hall thruster to generate a discharge; and a control unit for operating the power processing unit to provide to the Hall thruster a power pulse of a pre-selected duration xcfx84 less than 0.1d3xcfx81/{dot over (m)} where d is the characteristic size of the, xcfx81 is the propellant density at standard conditions, and {dot over (m)} is the propellant mass flow rate. A propellant storage and delivery system is responsive to the control unit for providing a synchronized propellant pulse of pre-defined duration approximately the same as the pre-selected duration of the power pulse and coincident to the discharge chamber with the power pulse for enabling the Hall thruster to produce a discrete output impulse.
The invention also features a pulsed Hall thruster system including a Hall thruster having an operating electron source, a magnetic circuit, and a discharge chamber. A power processing unit fires the Hall thruster and the control unit operates the power processing unit to provide to the Hall thruster a power pulse of pre-selected duration xcfx84 less than 0.1d3xcfx81/{dot over (m)}, where d is the characteristic size of the thruster, xcfx81 is the propellant density at standard conditions, and {dot over (m)} is the propellant mass flow rate. A propellant storage and delivery system is responsive to the control unit for providing a steady state supply of propellant to the discharge chamber for enabling the Hall thruster to produce a discrete output impulse.
The invention also features a pulsed Hall thruster system including a Hall thruster having an operating electron source, a magnetic circuit and a discharge chamber. A power processing unit fires the Hall thruster and a control unit operates the power processing unit to provide a continuous discharge voltage to the Hall thruster. A propellant storage and delivery system is responsive to the control unit for providing a propellant pulse of a pre-defined duration xcfx84 less than 0.1d3xcfx81/{dot over (m)}, where d is the characteristic size of the thruster, xcfx81 is the propellant density at standard conditions and {dot over (m)} is the propellant mass flow rate for enabling the Hall thruster to produce a discrete output impulse.
In preferred embodiments the Hall thruster may include a propellant conduit system with an input port and an output port proximate to the discharge chamber and the propellant storage and delivery system may include a propellant accumulator proximate the input port and a valve between the accumulator and the input port. The accumulator may maintain approximately constant pressure in the propellant conduit system and the propellant may provide the synchronized propellant pulse to the discharge chamber. The pre-selected duration of the firing pulse and the pre-defined duration of the propellant pulse may be the same. The valve may be integral with the Hall thruster to reduce discharge chamber filling time. The power pulse width may be variable and the control unit may set the width of the power pulse. The power pulse repetition rate may be variable and the control unit may set the repetition rate. The power processing unit may include a capacitor for supplying the power of the power pulse. The magnetic circuit may be segmented to reduce eddy currents and reduced magnetic flux rise time. The magnetic circuit may have high electrical resistivity to reduce eddy currents and reduce magnetic field rise time. The propellant conduit system fill time may be approximately equal to the magnetic rise time of the magnetic circuit. The magnetic circuit may include an electromagnet. The electromagnet may be energized in series with the Hall thruster discharge. The electromagnet may be energized by an independent source which can lead or lag the Hall thruster discharge. The magnetic circuit may include a permanent magnet.